1. Field of Invention
The present invention relates to a driving circuit that drives an active-matrix display device employing organic electroluminescent elements, and electronic equipment and an electro-optical device, incorporating the driving circuit. More particularly, the invention relates to a driving circuit having the function of applying a reverse-bias voltage to an organic electroluminescent element that controls degradation of the organic electroluminescent element, and electronic equipment and an electro-optical device, incorporating the driving circuit.
2. Description of Related Art
It is known that an organic EL display device is produced by arranging a plurality of pixels of organic electroluminescent elements, i.e., one of electro-optical elements, in a matrix. The organic electroluminescent element includes a metal electrode of Mg:Ag or Al:Li, etc., as a cathode, a transparent electrode, such as an ITO (Indium Tin Oxide) as an anode, and an organic thin-film laminate, including an emission layer, interposed between the cathode and the anode.
FIG. 9 shows a typical driving circuit for an active matrix display device employing an organic electroluminescent element. Referring to FIG. 9, the organic electroluminescent element is shown as a diode 10. The driving circuit is composed of transistors Tr 1 and Tr 2, each constructed of a thin-film transistor (TFT), and a capacitive element 2 storing charge.
Both the transistor Tr 1 and the transistor Tr 2 are of a p-channel TFT. The conduction state of the transistor Tr 1 is controlled in response to a charge stored in the capacitive element 2 as shown. The capacitive element 2 is charged through a data line VDATA by the transistor Tr 2, which is turned on with a selection voltage VSEL driven low. When the transistor Tr 1 is turned on, a current flows through the electroluminescent element 10 from the transistor Tr 1. With the current flowing therethrough, the electroluminescent element 10 continuously emits light.
FIG. 10 shows a simple timing diagram for the circuit shown in FIG. 9. When data is written, the selection voltage VSEL is driven low as shown FIG. 10. The transistor Tr 2 is turned on, thus charging the capacitive element 2. The charge duration is a write period TW. An actual presentation period follows the write period TW. During the presentation period, charge stored in the capacitive element 2 controls the conduction state of the transistor Tr 1. The presentation period is represented by TH as shown.
FIG. 11 shows another arrangement of the organic electroluminescent element driving circuit. The driving circuit shown is described in a paper entitled “The Impact of Transient Response of Organic Light Emitting Diodes on the Design of Active Matrix OLED Displays” (1998 IEEE IEDM98-875). Referring to FIG. 11, there are shown a driving transistor Tr 1, a charge control transistor Tr 2, a first selection transistor Tr 3, and a second selection transistor Tr 4 which is turned off during a charging period of the capacitive element 2.
As is known, transistors, even complying with the same specifications, suffer variations in performance. Even when the same voltage is applied to the gates of transistors, these transistors do not necessarily permit current of equal values to flow therethrough. Such variations cause nonuniformity in brightness. In the driving circuit, a current source 4 feeds a write current corresponding to a data signal, and the data signal thus controls the gate voltage of the transistor. In this way, the emission state of the organic electroluminescent element is controlled.
Transistors Tr 1 through Tr 4 are all of a p-type channel transistor. When the selection voltage VSEL is driven low, the transistors Tr 2 and Tr 3 are turned on, thereby storing, in the capacitive element 2, a charge corresponding to the value of the output of the current source 4. When the selection voltage VSEL is driven high, the transistors Tr 2 and Tr 3 are turned off. The conduction state of the transistor Tr 1 is thus controlled by the charge stored in the capacitive element 2. With a data hold control signal Vgp turning on the transistor Tr 4, an electroluminescent element 10 is supplied with a current corresponding to the charge stored in the capacitive element 2. This duration of time is a presentation period TH.
FIG. 12 shows a simple timing diagram of the circuit shown in FIG. 11. When data writing is performed by the current source 4 as shown in FIG. 12, the selection voltage VSEL is driven low, thereby turning on the transistors Tr 2 and Tr 3. The capacitive element 2 is thus charged. The charge period equals a write period TW. With the selection voltage VSEL driven high, the transistors Tr 2 and Tr 3 are turned off. With the data hold control signal Vgp driven low, the conduction state of the transistor Tr 1 is determined based on the charge stored in the capacitive element 2. The electroluminescent element 10 is supplied with a current corresponding to the charge stored in the capacitive element 2. This duration of time is a presentation period TH.
FIG. 13 shows another driving circuit of an organic electroluminescent element. The driving circuit shown here is disclosed in Japanese Unexamined Patent Application Publication No. 11-272233. As shown, the driving circuit includes a driving transistor Tr 1 which supplies an electroluminescent element 10 with a current from a power source during the on state thereof, a capacitive element 2 which stores a charge for controlling the conduction state of the transistor Tr 1, and a charge control transistor Tr 5 which controls the charging of the capacitive element 2 in response to an external signal. A voltage Vrscan is driven low to turn off a charge control transistor Tr 7 to cause the electroluminescent element 10 to emit light, and then, a reset signal Vrsig is not output. A transistor Tr 6 is included for adjustment purposes.
When the electroluminescent element 10 emits light in the driving circuit, the transistor Tr 5 is turned on, and the capacitive element 2 is charged by a transistor Tr 6 through a data line VDATA. Conductance between the source and drain of the transistor Tr 1 is controlled in response to a charge level of the capacitive element 2 to allow a current to flow through the electroluminescent element 10. Referring to FIG. 14, when the voltage Vscan is driven high to turn on the transistor Tr 5, the capacitive element 2 is charged through the transistor Tr 6. Conductance between the source and drain of the transistor Tr 1 is controlled in response to a charge level of the capacitive element 2 to allow a current to flow through the electroluminescent element 10.
Reverse-biasing the organic electroluminescent element is known as an effective way to prolong the service life of the organic electroluminescent element. For example, Japanese Unexamined Patent Application Publication No. 11-8064 discloses a technique for prolonging the service life of the organic electroluminescent element.
To reverse-bias the organic electroluminescent element using the disclosed technique, an additional power source for a negative voltage needs to be prepared and controlled to reverse-bias the organic electroluminescent element.